The individuals are always multicellular, the cells firmly bound
together and united into unbranched or branched filaments, expansions,
or masses of cells which grow by intercallary divisions or have
apical growth. In the first seven orders the cells are uninuclear,
but the cells of the remaining three orders contain several nuclei.
Asexual reproduction by zoospores, akinetes or aplanospores. Sexual
reproduction by isogamous or oogamous fertilisation.
The Confervoideæ, through the Ulvaceæ, are connected with the
Tetrasporaceæ, and from the _Coleochætaceæ_, which is the most
highly developed order, there are the best reasons for supposing
that the Mosses have taken their origin. The _Cladophoraceæ_
show the nearest approach to the _Siphoneæ_.
Order 1. =Ulvaceæ.= The thallus consists of one or two layers of
parenchymatous cells, connected together to form either a flat membrane
(_Monostroma_, _Ulva_) or a hollow tube (_Enteromorpha_), and may be
either simple, lobed, or branched. Reproduction takes place by detached
portions of the thallus; or asexually by zoospores or akinetes.
Gamete-conjugation is known to take place in some members of this
order, the zygote germinating without any resting-stage. The majority
are found in salt or brackish water.
[Illustration: FIG. 52.--_Ulothrix zonata_: a portion of a
filament with zoospores, which are formed two in each cell
(zoosporangium); the dark spots are the red “eye-spots”; 1,
2, 3, 4, denote successive stages in the development of the
zoospores; _b_ a single zoospore, _v_ the pulsating vacuole;
_c_ portion of a filament with gametes, sixteen are produced in
each gametangium; _d_ free gametes, solitary or in the act of
conjugation; _e_ the conjugation is completed, and the formed
zygote has assumed the resting-stage.]
Order 2. =Ulothricaceæ.= The thallus consists normally of a simple
unbranched filament (sometimes a small expansion consisting of one
layer of cells is formed, as in _Schizomeris_ and _Prasiola_ which
were formerly described as separate genera). Asexual reproduction
takes place by means of zoospores (with 1, 2, or 4 cilia), akinetes
or aplanospores; the last named may germinate immediately, or only
after a period of rest. Sexual reproduction takes place by the
conjugation of gametes of about the same size, each having two cilia
(Fig. 52 _d_). The zygote of _Ulothrix_, on germination, produces a
brood of zoospores which swarm for a time and then elongate to become
_Ulothrix_-filaments (alternation of generations). The gametes may also
germinate without conjugation in the same manner as the zoospores.
The principal genera are: _Ulothrix_, _Hormidium_, _Conferva_,
_Microspora_.--_Ulothrix zonata_ is very common in running fresh water.
Nearly all the species of _Hormidium_ occur on damp soil, tree-stems
and stones.
Order 3. =Chætophoraceæ.= The thallus consists of a single, branched,
erect or creeping filament of cells, often surrounded by mucilage.
The cells have only one nucleus. Asexual reproduction by zoospores
with 2 or 4 cilia, by akinetes, or aplanospores. In many, conjugation
between gametes with 2 cilia may be found. They approach on one side,
Ulothricaceæ, and on the other, Mycoideaceæ. The principal genera
are: _Stigeoclonium_, _Draparnaldia_, _Chætophora_, _Entoderma_,
_Aphanochæte_, _Herposteiron_, _Phæothamnion_, _Chlorotylium_,
_Trichophilus_, _Gongrosira_, _Trentepohlia_. Most of the species of
_Trentepohlia_ are coloured red by the presence of a red colouring
material, which occurs in addition to the chlorophyll. They are aerial
Algæ which live on stones (_T. jolithus_, “violet stone,” so named
on account of its violet-like odour in rainy weather), on bark and
old wood (_T. umbrina_), or on damp rocks (_T. aurea_). _Trichophilus
welckeri_ lives in the hair of Bradypus.
Order 4. =Mycoideaceæ.= The thallus is discoid, consisting of one or
more cell-layers, and is always attached. Asexual reproduction by
zoospores with 2 or 4 cilia. Sexual reproduction in some species by the
conjugation of gametes with 2 cilia. This order forms the connecting
link between _Chætophoraceæ_ and _Coleochætaceæ_. The species occur in
fresh water (_Chætopeltis_) as well as in salt (_Pringsheimia_), on the
carapace of tortoises (_Dermatophyton_ = _Epiclemmydia_), or endophytic
between the cuticle and the epidermal cells of the leaves of tropical
plants, destroying the leaf-tissue (_Mycoidea_).
Order 5. =Cylindrocapsaceæ.= The thallus consists of a simple (rarely,
in parts, formed of many rows) unbranched filament, attached in the
young condition, which has short cells with a single nucleus, and is
enveloped in a thick envelope with a laminated structure. Asexual
reproduction by zoospores with 2 cilia, which are formed 1, 2, or 4 in
each vegetative cell. The antheridia are produced by a single cell,
or a group of cells, in a filament, dividing several times without
increasing in size. Two egg-shaped spermatozoids, each with 2 cilia
(Fig. 53 _D_), are formed in each antheridium, and escape through
an aperture in the side; in the first stages they are enclosed in a
bladder-like membrane (Fig. 53 _B_, _C_). Other cells of the filament
swell out and form oogonia (Fig. 53 _A_), which resemble those of
_Œdogonium_. After fertilisation, the oospore surrounds itself with a
thick wall, and assumes a reddish colour. The germination is unknown.
The unfertilised oospheres remain green, divide often into 2–4
daughter-cells, and grow into new filaments.
[Illustration: FIG. 53.--_Cylindrocopsa involuta. A_ Oogonium
with oosphere (_o_) surrounded by spermatozoids (_s_). _B_ Two
antheridia, each with two spermatozoids. _C_ Spermatozoids
surrounded by their bladder-like membrane. _D_ Free spermatozoid.]
This order, which only includes one genus, _Cylindrocapsa_, forms the
connecting link between _Ulothricaceæ_ and _Œdogoniaceæ_. The few
species (4) occur only in fresh water.
Order 6. =Œdogoniaceæ.= The thallus consists of branched (_Bulbochæte_)
or unbranched (_Œdogonium_) filaments, attached in the early stages.
The cells may be longer or shorter, and have one nucleus. Asexual
reproduction by zoospores, which have a chaplet of cilia round the base
of the colourless end (Fig. 6 _a_). Sexual reproduction takes place by
oogamous fertilisation. On the germination of the oospore, 4 zoospores
are formed (Fig. 54 _F_). They occur only in fresh or slightly brackish
water. The division of the cells takes place in quite a peculiar and
unusual manner. At the upper end of the cell which is about to divide,
a ring-shaped thickening of soft cellulose is formed transversely round
the wall; the cell-nucleus of the mother-cell and the protoplasm then
divide by a transverse wall into two portions of similar size, and the
cell-wall bursts transversely along the central line of the thickened
ring. The cell-wall thus divides into two parts--the upper one short,
the “cap,” and the lower one much longer, the “sheath.” The portions
of the original cell-wall now separate from each other, the cellulose
ring extending, and supplying an additional length of cell-wall between
them. The cap and sheath will project a little in front of the piece
thus inserted. The dividing wall between the two new cells is formed
near to the uppermost edge of the sheath, and gradually becomes thicker
and firmer. The inserted piece of wall forms the larger part of the
wall of the upper cell: the remainder is formed by the cap. This mode
of division is repeated exactly in the same way, and new caps are
formed close below the first one, one for every division.
[Illustration: FIG. 54.--_A Œdogonium ciliatum. A_ Female
plant with three oogonia (_og_) and dwarf-males (_m_). _B_ An
oogonium with spermatozoid (_z_) seen entering the oosphere
(_o_) having passed through an aperture near the summit of the
oogonium; _m_ dwarf-male. _C_ Ripe oospore. _D Œdogonium
gemelliparum. F_ Portion of a male filament from which
spermatozoids (_z_) are emerging. _E_ Portion of filament of
_Bulbochæte_; the upper oogonium still encloses the oospore, in
the central one the oospore is escaping while the lower one is
empty. _F_ Four zoospores developed from an oospore. _G_ Zoospore
germinating.]
Fertilisation takes place in the following way. The oogonium is a
large ellipsoidal, swollen cell (_og_, in Fig. 54 _A_), whose contents
are rounded off into an oosphere with a colourless receptive-spot
(see _B_); an aperture is formed in the wall of the oogonium, through
which the spermatozoids are enabled to enter (_B_). The spermatozoids
are produced either directly, as in _D_ (in pairs), in basal cells
of the filament, or indirectly. In the latter case a swarmspore
(_androspore_) is formed which comes to rest, attaches itself to an
oogonium, germinates, and gives rise to a filament of a very few
cells--_dwarf-male_ (_A_, _B_, _m_). The spermatozoids are formed in
the upper cell of the dwarf-male (_m_), and are set free by the summit
of the antheridium lifting off like a lid. On the germination of the
oospore (_C_), which takes place in the following spring, 4 zoospores
are produced (_F_) (_i.e._ the sexual generation); these swarm about
for a time, and ultimately grow into new filaments.
[Illustration: FIG. 55.--_Coleochæte pulvinata. A_ A portion
of a thallus with organs of reproduction; _a_ oogonium before,
_b_ after fertilisation; _c_ an antheridium, closed; _d_ open,
with emerging spermatozoid. _B_ Ripe oogonium, with envelope. _C_
Germination of the oospore. _D_ Zoospore. _E_ Spermatozoid.]
Order 7. =Coleochætaceæ.= The thallus is always attached, and of a
disc- or cushion-shape, formed by the dichotomous branching of filaments
of cells united in a pseudo-parenchymatous manner. Each cell has only
one nucleus. Asexual reproduction by zoospores with 2 cilia (Fig. 55
_D_), which may arise in all the cells. Sexual reproduction by oogamous
fertilisation. The spermatozoids resemble the swarmspores, but are
smaller (_E_), and originate singly (in the species figured) in small
conical cells (_c_, _d_ in _A_). The oogonia are developed at the
extremities of certain branches: they are bottle-shaped cells with very
long and thin necks (_trichogyne_), open at the end (_a_ in _A_); at
the base of each oogonium is a spherical oosphere. The spermatozoids
reach the oosphere through the trichogyne, or through an aperture in
the wall when the trichogyne is absent, and fertilisation having taken
place, the oogonium becomes surrounded by a cell-layer (envelope),
which grows out from the cells near its base (_b_ in _A_), and in this
way a kind of fruit is formed (_B_) (_spermocarp_, _cystocarp_).
The oospore, next spring, divides and forms a parenchymatous tissue
(homologous with the Moss-sporophyte); this bursts open the envelope
(_C_), and a zoospore (homologous with the spores of the Moss-capsule)
arises in each of the cells, and produces a new _Coleochæte_. We have
then, in this case, a still more distinct alternation of generations
than in _Œdogonium_. Only one genus, _Coleochæte_, is known, but it
contains several species, all living in fresh water.
Order 8. =Cladophoraceæ.= This order is probably derived from the
Ulothricaceæ. The thallus consists of a single, unbranched or branched
filament, generally with an apical cell. The cells have each 2 or more
nuclei. Asexual reproduction by zoospores with 2 or 4 cilia, and by
akinetes. Conjugation of gametes with 2 cilia is found in some genera.
They occur in salt as well as in fresh water. The principal genera are:
_Urospora_, _Chætomorpha_, _Rhizoclonium_, _Cladophora_; of the last
named genus the species _C. lanosa_ and _C. rupestris_ are common in
salt water; _C. fracta_ and _C. glomerata_ in fresh water.
Order 9. =Gomontiaceæ.= _Gomontia polyrrhiza_, the only species
hitherto known, is found on old calcareous shells of certain salt water
Molluscs.
Order 10. =Sphæropleaceæ.= The thallus consists of free, unbranched
filaments, with very elongated multinuclear cells. The vegetative
cells form no zoospores. Sexual reproduction by oogamous fertilisation
(see page 13, Fig. 10 _B_). The oospore has a thick wall (Fig. 10
_D_) studded with warts, and assumes a colour resembling red lead. It
germinates only in the following spring, and produces 1–8 zoospores,
each with 2 cilia (Fig. 10 _E_), which grow into new filaments. Only
one species, _Sphæroplea annulina_, is known.
Family 3. Siphoneæ.
The thallus has apical growth, and in the vegetative condition consists
generally of one single (in the Valoniaceæ most frequently of more)
multinuclear cell, which may be much branched, and whose separate parts
in the higher forms (_e.g. Bryopsis_, Fig. 57; _Caulerpa_, Fig.
59, etc.) may be differentiated to perform the various physiological
functions (as root, stem and leaf). Vegetative multiplication by
detached portions of the thallus (gemmæ); asexual reproduction
by zoospores, akinetes, or aplanospores. Sexual reproduction by
gamete-conjugation, rarely by oogamous fertilisation. The zygote or
oospore germinates as a rule without any resting-stage.
[Illustration: FIG. 56.--_Botrydium granulatum_: _a_ an entire
plant forming swarmspores; _b_ swamspores; _c_ an individual with
gametangia; _d_, gamete; _e_, _f_, _g_ conjugation; _h_ zygote
seen from above; _i_ the same in a lateral view.]
Most of the Siphoneæ occur in salt water or on damp soil. Many (_e.g._
_Dasycladaceæ_) are very much incrusted with lime, and occur, in the
fossilized condition, in the deposits from the Cretaceous period to
the present time. The Siphoneæ are connected by their lowest forms
(_Botrydiaceæ_ or _Valonia_) with the Protococcaceæ, but show also,
through the Valoniaceæ, points of relationship to the _Cladophoraceæ_.
Order 1. =Botrydiaceæ.= The thallus in the vegetative condition
is unicellular, club-shaped, with a small single (_Codiolum_) or
repeatedly dichotomously branched system of colourless rhizoids
(_Botrydium_, Fig. 56 _a_), by which it is attached to objects immersed
in salt water (_Codiolum_) or to damp clay soil (_Botrydium_). Asexual
reproduction by zoospores with one (_Botrydium_) or two cilia, and by
aplanospores. The sexual reproduction is only known in _Botrydium_, and
takes place in the following manner: in the part of the thallus which
is above ground and in an active vegetative condition, several round
cells (Fig. 56 _c_) are formed, which may be green or red according as
they grow under water, or exposed to the strong light of the sun. These
cells must be considered as “gametangia” as they produce many gametes
(_d_) provided with two cilia. The zygote (_h_, _i_) formed by the
conjugation (_e_, _f_, _g_) may either germinate immediately, or become
a thick-walled resting-cell of an irregular, angular form.
Order 2. =Bryopsidaceæ.= The thallus in the vegetative condition
is unicellular, and consists at the lower extremity of branched
rhizoids, while the upper portion is prolonged into a stem-like
structure of unlimited growth, producing, acropetally, branches
and leaf-like structures. The latter have limited growth,
and are separated by a cross wall from the stem, and become
gametangia, or drop off. The gametes have two cilia, and are of
two kinds: the female, which are green and large and the male,
which are of brownish colour and smaller. Zoospores or any other
method of asexual reproduction are unknown. Only one genus,
_Bryopsis_, living in salt water.
[Illustration: FIG. 57.--_Bryopsis plumosa_. A the plant, natural
size. B A portion (enlarged) which shows the growing point (v),
and the leaves derived from it in acropetal succession.]
Order 3. =Derbesiaceæ.= Only one genus, _Derbesia_, living in
saltwater. The zoospores, which are formed in a few lateral,
swollen zoosporangia, possess one nucleus which has arisen
through the coalescence of several, and they resemble the
zoospores of _Œdogonium_ by having a circle of cilia attached at
the base of the colourless spot.
Order 4. =Vaucheriaceæ.= The thallus consists, in the vegetative
condition, of a single irregularly or dichotomously branched cell,
without differentiation into stem or leaf; root-like organs of
attachment may however occur. Asexual reproduction by zoospores, which
are formed singly in the extremity of a branch cut off by a transverse
wall. They contain many nuclei, and bear small cilia situated in
pairs, which give the appearance of a fine “pile” covering the whole
or a great part of the surface. Akinetes, aplanospores, and phytoamœbæ
(naked masses of protoplasm, without cilia, which creep like an amœba
on a substratum) may occur under certain conditions.
The sexual reproductive organs are formed on short lateral branches,
and are separated from the vegetative cell (Fig. 58 _A_) by cell-walls.
Numerous spermatozoids, each with two cilia, are developed in the
coiled antheridium (_A_, _b_). The oogonium is a thick, egg-shaped,
often oblique cell, with its protoplasm rounded into an oosphere,
which has a hyaline “receptive-spot” (_A_, _a_) immediately beneath
the aperture formed in the wall of the oogonium. A slimy mass, which
serves to receive the spermatozoids, is formed in some species in this
aperture. The spermatozoids when liberated swim towards and enter
the oosphere, which then immediately surrounds itself with a thick
cell-wall. The mature oospore (_B_) contains a large quantity of oil.
At germination the outer cell-wall bursts and a new plant is formed.
There is only one genus, _Vaucheria_, with species living in salt as
well as in fresh water and on damp soil.
[Illustration: FIG. 58.--_Vaucheria sessilis_. _A_ Fertilisation;
_b_ the antheridia; _a_ the oogonia; _a_ the receptive spot. _B_
Oospore.]
Order 5. =Phyllosiphonaceæ= are parasites in the leaves and stalks of
Flowering-plants.
Order 6. =Caulerpaceæ.= The thallus has distinct differentiation into
root, stem and leaf-like members (Fig. 59); it is unicellular. Within
the cell, strong, branched threads of cellulose extend from one side
to the other serving as stays to support the thallus. Reproduction
takes place by detached portions of the thallus; no other modes of
reproduction are known. This order may most approximately be classed
with the _Bryopsidaceæ_. The genus _Caulerpa_ consists of more than
seventy species which inhabit the tropical seas.
Order 7. =Codiaceæ.= The thallus has various forms, but without
distinct differentiation in stem- or leaf-structures, sometimes (_e.g._
_Halimeda_) it is very much incrusted with lime. In the early stages
it is unicellular (later, often multicellular), very much branched,
with the branches, at any rate partly, so united or grown in amongst
one another (Fig. 60) that an apparently parenchymatous cellular
body is formed. Akinetes or aplanospores are wanting; zoospores (or
gametes?) may be developed in some species, however, in special swollen
sporangia. Fertilisation similar to that in _Bryopsis_ occurs perhaps
in _Codium_. They are all salt water forms.
Order 8. =Valoniaceæ.= The thallus is generally multicellular, without
differentation into stem- or leaf-structures, but the cells are
sometimes united together and form a leaf-like reticulate expansion
(_e.g. Anadyomene_). Zoospores are known in some, and they are then
formed directly in the vegetative cells. In others (_e.g. Valonia_),
a mass of protoplasm, which maybe separated through the damaging of
a cell, can surround itself with a cell-wall, and grow into a new
plant. No other modes of reproduction are known. The most important
genera are: _Valonia_, _Siphonocladus_, _Chamædoris_, _Struvea_,
_Microdictyon_, _Anadyomene_. They are all salt water forms.
[Illustration: FIG. 59.--_Caulerpa prolifera_ (natural size).]
As already pointed out, the _Valoniaceæ_ occupy a somewhat
central position among the Siphoneæ, and present points
of similarity and contrast with the _Botrydiaceæ_ and the
_Bryopsidaceæ_ through _Valonia_, with the _Dasycladaceæ_
through _Chamædoris_, and also with the _Cladophoraceæ_ through
_Siphonocladus_, and _Struvea_.
Order 9. =Dasycladaceæ.= The thallus consists of an axile longitudinal
cell, destitute of transverse walls, attached at the base by root-like
organs of attachment, and producing acropetally whorls of united,
single or branched, leaf-like structures with limited growth. Asexual
reproduction is wanting. Sexual reproduction by conjugation of gametes
which arise in separate, fertile leaves, either directly or from
aplanospores, which develope into gametangia. The principal genera are:
_Acetabularia_, _Dasycladus_, _Neomeris_, _Cymopolia_. All marine.
[Illustration: FIG. 60.--_Halimeda opuntia._ Plant (natural
size). _B_ Part of a longitudinal section.]
The curiously shaped _Acetabularia mediterranea_ grows gregariously
on limestone rocks, and shells of mussels in the Mediterranean; it
resembles a minute umbrella with a small stem, sometimes as much as
nine centimetres in height, and a shade which may be more than one
centimetre in diameter. The cell-membrane is thick, and incrusted
with carbonate and oxalate of lime. Only the lower, root-like part of
the thallus, which penetrates the calcareous substratum survives the
winter, and may grow up into a new plant. The sterile leaves, which
drop off early, are dichotomously branched and formed of cylindrical
cells separated from each other by cross-walls, but they are not grown
together. The shade is formed by a circle of 70–100 club-shaped rays
(fertile leaves) grown together, in each ray 40–80 aplanospores are
formed, which become liberated at the breaking of the shade, and later
on are changed to gametangia (compare _Botrydium_) which open by a
lid and allow a large number of egg-shaped gametes with two cilia to
escape. Gametes from various gametangia conjugate with one another; the
product of the conjugation swarms about for some time, rounds off, and
then surrounds itself with a cell-wall. The zygote germinates after
a period of rest and then produces a sexual plant. The aplanospores
(gametangia) thus represent the sexual generation.